'''
Design your implementation of the linked list. You can choose to use the singly linked list or the doubly linked list. A node in a singly linked list should have two attributes: val and next. val is the value of the current node, and next is a pointer/reference to the next node. If you want to use the doubly linked list, you will need one more attribute prev to indicate the previous node in the linked list. Assume all nodes in the linked list are 0-indexed.

Implement these functions in your linked list class:

.get(index) : Get the value of the index-th node in the linked list. If the index is invalid, return -1.
.addAtHead(val) : Add a node of value val before the first element of the linked list. After the insertion, the new node will be the first node of the linked list.
.addAtTail(val) : Append a node of value val to the last element of the linked list.
.addAtIndex(index, val) : Add a node of value val before the index-th node in the linked list. If index equals to the length of linked list, the node will be appended to the end of linked list. If index is greater than the length, the node will not be inserted.
.deleteAtIndex(index) : Delete the index-th node in the linked list, if the index is valid.

Example:

Input: 
["MyLinkedList","addAtHead","addAtTail","addAtIndex","get","deleteAtIndex","get"]
[[],[1],[3],[1,2],[1],[1],[1]]
Output:  
[null,null,null,null,2,null,3]

Explanation:
MyLinkedList linkedList = new MyLinkedList(); // Initialize empty LinkedList
linkedList.addAtHead(1);
linkedList.addAtTail(3);
linkedList.addAtIndex(1, 2);  // linked list becomes 1->2->3
linkedList.get(1);            // returns 2
linkedList.deleteAtIndex(1);  // now the linked list is 1->3
linkedList.get(1);            // returns 3

Constraints:

0 <= index,val <= 1000
Please do not use the built-in LinkedList library.
At most 2000 calls will be made to get, addAtHead, addAtTail,  addAtIndex and deleteAtIndex.
'''

# Singly Linked List

class ListNode(object):

    def __init__(self, x):
        self.val = x
        self.next = None


class MyLinkedList(object):

    def __init__(self):
        """
        Initialize your data structure here.
        """
        self.size = 0
        self.head = ListNode(0)  # sentinel node as pseudo-head

        

    def get(self, index):
        """
        Get the value of the index-th node in the linked list. If the index is invalid, return -1.
        :type index: int
        :rtype: int
        """
        # if index is invalid 
        if index < 0 or index >= self.size:
            return -1

        curr = self.head  # move the point to the head
        # index steps needed to move from sentinel node to wanted index
        for _ in range(index + 1):
            curr = curr.next
        
        return curr.val

        

    def addAtHead(self, val):
        """
        Add a node of value val before the first element of the linked list. After the insertion, the new node will be the first node of the linked list.
        :type val: int
        :rtype: None
        """
        self.addAtIndex(0, val)
        

    def addAtTail(self, val):
        """
        Append a node of value val to the last element of the linked list.
        :type val: int
        :rtype: None
        """
        self.addAtIndex(self.size, val)
        

    def addAtIndex(self, index, val):
        """
        Add a node of value val before the index-th node in the linked list. If index equals to the length of linked list, the node will be appended to the end of linked list. If index is greater than the length, the node will not be inserted.
        :type index: int
        :type val: int
        :rtype: None
        """
        # if index is greater than the length, the node will not be inserted
        if index > self.size:
            return
        
        # if index value is negative, the node will be inserted at the head
        if index < 0:
            index = 0
        self.size += 1

        # find predecessor of the node to be added
        pred = self.head
        for _ in range(index):
            pred = pred.next
            
        # node to be added
        to_add = ListNode(val)
        # insertion itself
        to_add.next = pred.next
        pred.next = to_add


    def deleteAtIndex(self, index):
        """
        Delete the index-th node in the linked list, if the index is valid.
        :type index: int
        :rtype: None
        """
        # if the index is invalid
        if index < 0 or index >= self.size:
            return
        
        self.size -= -1
        # find predecessor of the node to be deleted
        pred = self.head
        for _ in range(index):
            pred = pred.next

        # delete pred.next
        pred.next = pred.next.next
        


# Your MyLinkedList object will be instantiated and called as such:
if __name__ == "__main__":
    obj = MyLinkedList()
    print(obj.addAtHead(7))
    print(obj.addAtHead(2))
    print(obj.addAtHead(1))
    print(obj.addAtIndex(3, 0))
    print(obj.deleteAtIndex(2))
    print(obj.addAtHead(6))
    print(obj.addAtTail(4))
    print(obj.get(4))
    print(obj.addAtHead(4))
    print(obj.addAtIndex(5, 0))
    print(obj.addAtHead(6))

